Various systems for automatically guiding a workpiece through a machine have been proposed in the prior art. For example, U.K. Patent Specification No. 1,063,001 describes a sewing machine in which the axis of a guide wheel is turned with the aid of a cam controlled linkage so as to cause the workpiece to move along a desired path through the machine. This arrangement requires a special cam to be constructed every time the path of the workpiece has to be changed, and there is no means to ensure accurate steering of the workpiece along the required path.
U.K. Patent Specification No. 1,205,290 discloses a sewing machine in which a combined guide/feed wheel is controlled, for the purpose of steering the workpiece, by a servo mechanism which includes a sensor sensing the edge of the workpiece with a view to providing a stitch line which remains at a uniform distance from the edge. However, since the combined feed/guide wheel is turned to provide guidance, the actual direction of feeding of the workpiece through the machine will be varied. Highly accurate guidance is not ensured and there is no means to ensure that the actual stitches formed remain parallel to the edge of the workpiece.
U.K. Patent Specification No. 962,160 discloses a machine for performing a folding operation on the edge of a workpiece, the machine including an automatic guidance mechanism. In the machine, the workpiece is advanced intermittently. Each advance of the workpiece is in a straight line. In the intervening periods, steering of the workpiece is effected. The steering mechanism comprises an edge sensor and a pair of spaced apart wheels having their axes perpendicular to the direction of advance but driven, in the aforesaid intervening periods and when commanded by the edge sensor, with different peripheral speeds which are in a fixed ratio to one another. Thus, the point about which the workpiece is turned is fixed, and is irrespective of the radius of curvature of the line which the workpiece is to follow; and furthermore the actual advance of the workpiece is always in a straight line.
U.K. Patent Specification No. 1,002,283 discloses a sewing machine in which the workpiece is advanced intermittently. The advance is always along a straight line. However, steering means is provided for effecting a steering action on the workpiece by causing the workpiece to rotate about the needle when the needle is inserted into the workpiece. This means is controlled by a servo mechanism including a photoelectric device for sensing the edge of the material. However, as indicated, the centre of turning is always the needle, and this is regardless of the radius of curvature. Also, since the advance of the material is always in a straight line, the actual stitches will not always be parallel to the edge.
U.K. Patent Specification No. 1,002,281 discloses a system similar to that in U.K. Patent Specification No. 1,002,283 except that the machine is a cementing and folding machine and in this case the centre of rotation is provided by a special gripper which engages the material at a fixed point in the periods between advance of the workpiece.
U.K. Patent Specification No. 989,874 discloses a skiving machine in which the workpiece is continuously advanced toward the skiving knife. An edge sensor and servo mechanism is provided for effecting steering of the workpiece with a view to causing the skiving to be carried out correctly along the edge. Rotation of the workpiece during steering takes place about arbitrary centres and the machine is only operable at a constant speed.
Accordingly, there exists a need for a guidance mechanism in which highly accurate guidance can be achieved and, if desired, variations in the rate of feeding the workpiece through the machine can be accommodated by the guidance apparatus, without loss of accuracy of the guiding operation.